1. Field of the Invention
The invention relates to adhesive room temperature vulcanizing (RTV) silicone rubber compositions (also called "compounds" herein) based on polydiorganosiloxanes, which cross-link by means of condensation, the compounds containing, in addition to the conventional components, the combination of at least one hydrolyzable alkylsilane and at least one branched polysiloxane.
2. Discussion of the Background
There exist silicone rubber compounds that vulcanize to elastomers at room temperature. They contain in general a cross-linkable polymeric, usually linear siloxane, a compound that has a vulcanizing effect, a catalyst and optionally other additives, like plasticizers, bonding agents, pigments, processing agents and fillers.
The very good adhesive properties of silicone rubbers are not always advantageous, since frequently a strong adhesion of the silicone on different substrates is to be achieved. Therefore, one uses so-called bonding agents, which are either integrated into the silicone rubber compound or are applied on the substrate as primer. Normally these bonding agents comprise one or several monomeric silanes, which exhibit functional groups bonded on the silicon via carbon. There also exist compounds that have the effect of a bonding agent and that are obtained through the reaction of different bonding agents.
Technically preferred is the integration of the bonding agents into the silicone rubber compounds, thus saving the additional step of applying the bonding agent. However, one important problem for the stability of the adhesive shear strength of such adhesive bonds or sealants lies in the fact that, on the one hand, the adhesion of the silicone on the substrate and, on the other hand, the vulcanizate strength, for example at elevated temperatures and subject to the simultaneous effect of moisture, declines very rapidly. Within a short period of time the result is a significant reduction of the technical service life of the combination silicone and substrate.
According to the DE 23 01 547 a combination of aminopropyl triethoxysilane and glycidoxypropyl trimethoxysilane or analogous alkoxysilanes with amino groups and epoxide groups are used for an adhesive bond of silicone with glass. EP 178 751 describes the integration of these compounds into the silicone rubber compound. These systems have the drawback that with a tensile load on such an adhesive bond following simultaneous stress by temperature and humidity (hot water storage) the adhesion tears.
Several methods have been tried to avoid this decrease in strength. Thus, the addition of colophonium esters in the form of a solution in toluene is described in the EP 540 540. JP 3 017 158 teaches the use of calcium carbonates coated with colophonium, thus necessitating an additional treatment of the fillers.